This invention relates to a multilayer mirror for soft X-rays, widely used in a scientific instrument or a manufacturing apparatus, such as an optical element of beam line of a synchrotron radiation, an X-ray microscope, and an X-ray exposure apparatus. Particularly, it relates to a multilayer mirror capable of being used even in a case of a normal incidence in which an incident angle is practically perpendicular to the mirror surface.
Generally, in almost all materials, refractive indices are substantially equal to one to a light having a shorter wavelength, as for example, the so-called vacuum ultraviolet light, and the absorption coefficients are more than negligible. As a result, a conventional reflection mirror fails to function properly.
In recent years, a method for depositing a multilayer film while controlling the thickness of each layer of a large number of thin films of the order of angstrom has been developed. By using that method, multilayer films have been proposed in which each reflecting light from each of a plurality of layer interfaces constructively interfere each other, and they are currently under active research and development.
In general, the multilayer mirror consists of a multilayer film in which tens to hundreds of thin-films made of different materials are deposited alternately. In order to obtain a high reflectivity, a combination of materials of the thin films must be selected so that a difference between the refractive indices of the materials adjacent to each other is as large as possible. A well-known rough criterion for selecting the combination of the materials is a material mainly consisting of a heavy element and a material consisting mainly of a light element.
Up to the present, the combinations of materials of the thin films consisting mainly of a heavy element such as silver (Ag), nickel (Ni), or cobalt (Co), and materials of a thin film mainly consisting of a light element such as carbon (C) and boron (B) are known in the cases in which the soft X-ray has a wavelength of around 100 .ANG. or less.
Co is particularly suitable for the material of the multilayer film for reflecting the light having a wavelength of 100 .ANG. or less because of a large difference in the refractive index between Co and the refractive index of a light element, and its relatively small absorption coefficient.
However, as reported by P. Ruterana (J. Appl. Pys., Vol. 68, p. 1033, 1990), when Co is formed into a thin film of 30 .ANG. or less, condensation of the material occurs. Accordingly, a uniform and continuous thin films cannot be obtained. As a result, in a multilayer film with a thickness of 30 .ANG. or less having Co thin film as a layer having smaller refractive index, the Co layers are formed in an island-like condition. This condition causes an increase in the roughness of the layer interfaces of the multilayer film and further increase of scattering of X-rays. Thus, the reflectivity of the multilayer film reduced, to the extent that, in a serious case, almost no reflection occurs.